This invention relates to an improved fused short circuit and grounding switch which is operable to clear low current arcing faults on high current high capacity electrical circuits, such as in protecting 480Y/277 volt electric service.
Historically, overcurrent protection on power systems has been designed to sense and clear so-called "bolted faults" in which the fault impedance is assumed to be zero. Such faults produce maximum fault currents that are easily distinguished from load and inrush currents, and overcurrent protective devices now in use are generally adequate for this purpose. On the other hand, sensing and clearing low current arcing faults, particularly on 480Y/277 volt service, has proven extremely difficult. For instance, although ground fault circuit interrupters now available respond to low current arcing faults on branch circuits, in a manner which complies with code requirements, they are not now generally used in the mains, since they inherently lack the required selectivity for coordination to prevent the unnecessary and undesirable tripping of tandemly connected ground fault circuit interrupter equipped devices.
Although the magnitude of the current in an arcing fault may be much less than in a bolted fault, arcing faults can be much more destructive, because they release tremendous energy concentrated at the fault location. In contrast, dissipation of the energy of a bolted fault is distributed throughout the system resistance on the source side of the fault.
Low current arcing faults may occur anywhere on a 480Y/277 volt system, and my fused short circuit and grounding switch may be employed at various locations therein as may be desired. However, the same most usually occur in the customer's equipment, such as the customer's main switchboard. Such faults can be initiated by a variety of causes, including inadvertently allowing fish tapes to enter equipment with bare buses, slippage of electricians' tools, loose connections, rodents, insulation contamination and deterioration, heat cycling, voltage surges, and mechanical effects of current surges. When a low current arcing fault occurs in the customer's switchboard, the load side thereof is not protected by the overcurrent type circuit breakers of the switchboard, and the line side thereof is now totally unprotected. Thus, when a low current arcing fault occurs in a customer's switchboard, it is not at all unusual for the entire switchboard to be destroyed.
Low current arcing faults also frequently occur in network protectors, in which case the network protector is likely to be destroyed, and other equipment interconnected therewith, such as a transformer, extensively damaged.
The problem of sensing and clearing low current arcing faults is presently sought to be resolved by two basic methods: opening of low-voltage network protectors, and high-voltage circuit interrupters. The disadvantage of using a low-voltage network protector is that the same may not sense and clear the fault if the fault occurs in the protector, and high-voltage circuit interrupters are too costly to use on all 480Y/277 volt services.
It is accordingly a primary object of this invention to provide a reliable and economically feasible fused short circuit and grounding switch for sensing and clearing low current arcing faults which may occur in such as a 480Y/277 volt service. Of course, my switch may be used in other than 480Y/277 volt services, and may be useful other than in sensing and clearing low current arcing faults, and use thereof is not limited to such purposes, such use thereof being set forth herein since my switch was principally developed for such purposes and as illustrative of an example of the usefulness thereof.
A further object of the invention is the provision of a fused short circuit and grounding switch having a high interrupting capability with a minimum of arcing; which may be incapsulated to be submersible, explosion proof, and effectively sealed against dust, moisting, rodents, or accidental contact with parts or material which could cause arcing and render the same ineffective; and which may be located remote from the device to be protected, a sensor being provided for the device to be protected and interconnected to my switch for selective operation thereof when a condition is created or exists in the device or area to be protected which calls for operation of my improved switch. In this regard, present code requirements limit the quantity and location of main switches, for the convenience of fire fighters. By providing relatively inexpensive, unrestricted, multiple remote tripping sensors, low current arcing fault protection is provided by such as thermal sensors in the switchboard, ground fault circuit interrupter protection can be provided by auxiliary relays, and fire fighter protection can be provided by a suitable manually operated emergency toggle switch in the building lobby. It would also be practicable to interconnect automatic sprinkler and alarm systems with the sensor circuit.